In general, packet-oriented methods are used for VODSL, in order to be able to offer the known telephony services (ISDN or analog) by way of xDSL lines. Methods known up to the present are:
I. Broadband Loop Emulation Services (BLES)
This solution is shown in FIG. 1. In this method, the subscriber signaling protocols (e.g., GR303, V5, ISDN) and the voice channel are transparently carried by a conventional subscriber switching center (Tln-Vst) to the line termination in the Integrated Access Device (IAD) at the end customer location, by way of the packet-oriented access network. In most cases, ATM/AAL2 is used as the transport protocol for the transmission of signal and voice. Other transport protocols, e.g., FR or IP, would also be conceivable.
Disadvantages of this solution approach are, among others:                Complicated, error-prone operation, because the subscriber must be set up twice, once at the subscriber switching center and again in the AN.        Restriction to voice services, i.e., cannot be expanded to multi-media, in spite of the use of a packet-oriented network.        
Advantages of this solution method are, among others:                Assuring conventional telephony services and subscriber terminations, conventional terminals can continue to be used without restrictions.        The quality of service (QOS) required for the telephony service is assured by using a dedicated ATM-PVC in the access network and TDM in the subscriber switching center.II. Voice Over Multi-Service Data Networks (VOMSDN)        
This solution is shown in FIG. 2. This method is based on VoIP and utilizes the corresponding protocols, e.g., H323, SIP, MGCP. Access to the conventional PSTN no longer takes place by way of a subscriber switching center, but rather by way of a transit switching center. VoIP controller and media gateway take over the interworking function with regard to signaling (H.323, SIP <-> ISUP) and working channel (VoIP <-> TDM).
Disadvantages of this solution method are, among others:                The quality of service (QOS) required for telephony service cannot be assured, since voice, signaling, and data packets are transported by way of the same ATM connection.        The VoIP telephony service being offered is not identical with conventional telephony service—conventional terminals can only be used within limitations.        
Advantages of this solution method are, among others:                The possibility of using LANs as a distribution network also for telephony services, at the end customer location.        Expandability to multi-media services, particularly when using H.323 or SIP.Alternative local network operators (e.g., COLT) who want to serve end customers directly must introduce new access technologies or lease the subscriber access line from the former market controller (e.g., Deutsche Telekom). The monthly leasing costs in this connection are in the range of the basic subscriber fee for conventional telephony service, so that the alternative local network operator must offer additional services by way of the leased subscriber access line, in order to be profitable.        
These additional services are based on the xDSL technology and increasingly consist of Voice_over_DSL (VODSL), in addition to broadband Internet access. Using VODSL, the broadband xDSL technology is utilized to make up to 16 subscriber connections for lucrative telephony services (ISDN or analog) available to the end customer, by way of one connection line. The target group for VODSL services is primarily small to medium-size businesses.